Trump Jr. also revealed that his father has suggested at moments that he tone down what he's posting on social media.
Donald Trump Jr. claimed Tuesday that the modern-day Democratic Party has moved so far left it would have rejected President John F. Kennedy as an "alt-right, neo-Nazi terrorist."
The president's son launched the broadside against Democrats during an interview on Fox News to promote his newly published book, "Triggered: How the Left Thrives on Hate and Wants to Silence Us."
"The reality is this isn't your grandfather's Democrat Party," Trump Jr. said in the interview. "If you look at their party platform, it's not for working-class Americans. You know, JFK would be an alt-right neo-Nazi terrorist, according to them today."
Donald Trump and his allies have repeatedly claimed that the Democratic Party is too liberal for the average American, deriding self-professed democratic socialist Bernie Sanders and liberal freshmen Democrats in the House, like New York's Alexandria Ocasio-Cortez and Minnesota's Ilhan Omar.
Trump Jr. did a round of television interviews to promote his new book a day after his father took to Twitter to urge his 66.5 million followers to "Go order it today!" Trump has accused former Vice President Joe Biden's son of profiting off his father's office.
Trump Jr. also revealed that his father, who frequently posts controversial comments on Twitter, has suggested at moments that he tone down what he's posting on social media.
"Every once while, I'll get that call like, 'Hey, you're getting a little hot on social,'" Trump Jr. said, recalling conversations with his father. "I go, 'Wait a minute.' I will take your advice. I will take your advice on anything. ... But I was like this may be the one place where I'm just going to say I'm on my own and maybe, you know, you don't have the authority to start talking about this."
Some ethics watchdogs say the president's promotion of his son's book raises red flags.
Trump's promotional tweet would be a violation of ethics rules if it had come from any federal employee other than the president, said Liz Hempowicz, the director of public policy at the Project on Government Oversight, a nonpartisan government watchdog group.
"Frankly he's using his Twitter account to try to financially benefit his son," she said Monday. "That's not only distasteful, but it's a misuse of public office and it would be an official misuse of public office if it was anyone other than the president."
The tweet also highlights a well-practiced tactic of Trump trying to turn a weakness into an attack on his opponents.
In this case, Trump has zeroed in on Biden's son Hunter, going as far as to ask foreign governments, including Ukraine and China, to investigate the Biden family's business dealings. Those efforts helped spark the impeachment inquiry into his conduct.
Biden is a leading contender for the Democratic nomination and could face Trump in the general election next year.
"The Biden family was PAID OFF, pure and simple!" Trump insisted in a tweet last month, despite no evidence suggesting that the former vice president received any payments or that Hunter Biden did anything illegal. The younger Biden has acknowledged he displayed poor judgment when he took a post on the board of a Ukrainian energy firm, Burisma, after his father became the Obama administration's point person on U.S.-Ukraine relations.
Hunter Biden also recently said that he would step down from the board of directors of a Chinese-backed private equity firm because his service had become a "distraction."
Trump is repeating the playbook he used during his 2016 campaign, when he tried to paint his Democratic rival Hillary Clinton as corrupt and untrustworthy, mixing legitimate criticism of her past with unfounded conspiracy theories.
Trump is the first president in modern history who has failed to divest from his business holdings. He makes frequent trips to his for-profit golf clubs, continues to collect dues at his members-only properties, and hosts fundraisers and foreign delegations at hotels that bear his family's name.
And his sons continue to operate his company, at one point trying to launch a lower-budget hotel chain they hoped would appeal to Trump voters.
The White House did not respond to questions Monday. But a spokesman for Trump Jr. defended the tweet, insisting that there is little in common between a father promoting his son's book and a son being paid large sums of money by foreign companies because of his last name.